1. Field of the Invention
The present invention relates generally to electronic image decompression. More particularly, the present invention relates to improvements in reconstructing images from compressed image data using estimated timing information.
2. Description of Prior Art
An increasing number of modern data processing applications utilize electronic document images in place of or in conjunction with paper documents. For example, electronic images of customer signatures may be used to authorize credit requests in electronic Point of Sale (POS) transactions. The electronic image of the customer signature is typically compressed and then transmitted to a remote central database for verification, storage, or other processing. If necessary, a physical record of the transaction may be readily generated from the electronic image at the remote site. In such transactions, it is often very important that the compressed electronic image, when reconstructed, is perceptually indistinguishable from the original. The reconstructed image may then be used, for example, to verify the authenticity of a customer signature. Similar authenticity concerns are present with many other types of electronic document images.
Image compression techniques, however, are typically directed to reducing the amount of digital data needed to represent a particular image, and often discard certain image information in the interest of data reduction. When the compressed data is subsequently decompressed, the reconstructed image may therefore exhibit large deviations from the original. In the case of a handwritten signature, the reconstructed image may not be sufficiently similar to the original to serve as a basis for verifying authenticity. U.S. Pat. No. 5,091,975, which is incorporated by reference herein, discloses an exemplary data compression technique.
One known compression technique involves using a plurality of linear segments to approximate the original image. The electronic image data points making up, for example, a signature are compressed by approximating a group of original image data points as a linear segment connecting a pair of compressed image data points, also referred to as anchor data points. The image is reconstructed by simply connecting each pair of compressed image data points by a linear segment. Linear segment approximation techniques can compress image data by a factor of as much as ten to one.
A significant problem with linear segment approximation techniques is that timing information, such as the number of original image data points between each pair of compressed image data points, is usually lost in the compression process. The reconstructed image is therefore often perceptually different from the original image. In the case of a signature, proper authentication may not be possible using the reconstructed image. Of course, a higher resolution reconstructed image may be obtained by using more compressed image data points. Additional image data, however, results in slower image retrieval, transmission and processing rates, higher memory capacity requirements for image storage, and higher equipment costs.
As is apparent from the above, a need exists for a method and apparatus for reconstructing compressed image data which provide improvements in reconstructed images without increasing the amount of compressed image data.